Observation of a charged charmoniumlike structure at BESIII

An international team of scientists that operate the BESIII Experiment at
the Beijing Electron Positron Collider in China recently began a series of
specialized studies aiming at an understanding of the anomalous "Y(4260)"
particle. As a striking and unexpected first observation from these new
studies, the collaboration has reported that the Y(4260) particle in fact
decays to a new, and perhaps even more mysterious, particle that they
named the "Zc(3900)."

Since its 2005 discovery by the BaBar experiment at the SLAC National
Laboratory in Stanford California, the Y(4260) particle has continued to
mystify researchers. While other particles that share certain similarities
to the Y(4260) have long been successfully explained as examples of a
charmed quark and anti-charmed quark paired together by the strong force
of particle physics, attempts to incorporate the Y(4260) into this model
have failed, and its underlying nature remains unknown.

In late December of 2012, the BESIII team embarked on a program of
research to produce large numbers of Y(4260) particles by annihilating
electrons and anti-electrons (positrons) with a total energy that
corresponds to the mass of the Y(4260). Once produced, the Y(4260) quickly
decays, and its decay products are measured with the BESIII particle
detector. According to Prof. Xiaoyan Shen, the spokesperson of the BESIII
experiment, from the Institute of High Energy Physics, Chinese Academy of
Sciences, "the goal of our program is to understand the various processes
by which the Y(4260) decays with the hope that this will provide clues
about its internal structure, and thereby yield new insights into the
workings of the strong force, which is responsible for holding quarks
together inside subatomic particles."

While commonly known subatomic particles, such as the proton and the
neutron, are comprised of the relatively lightweight up- and down- quarks,
the BESIII Experiment is specialized for the study of matter that contains
the heavier charmed quarks. The J/psi particle, for example, which is
known to be composed of a charmed quark and an anti-charmed quark bound
together by the strong force, can be copiously produced at the collider in
Beijing. "To date, BESIII has directly produced more than a billion J/psi
particles in these electron-positron annihilations," according to Prof.
Fred Harris from the University of Hawaii, the co-spokesperson of the
BESIII experiment. The J/psi particle forms the cornerstone of what has
been thought to be a well understood system of various possible
configurations of charmed and anti-charmed quarks, called the "charmonium"
mesons, that are the simplest and considered to be among the most easily
understood subatomic particles. But the recent discoveries of several new
particles, including the Y(4260) and now the Zc(3900), have cast doubt on
these optimistic assessments and suggest that more complex structures have
to be considered.

Previous studies of the Y(4260) used electron and positron beams with a
total energy that was well above that which corresponds to the mass of the
Y(4260). In these experiments, the Y(4260) mesons were produced via the
relatively rare process in which either the original electron or positron
beam particle first radiated a high energy gamma-ray, thereby lowering the
total annihilation energy to the Y(4260) mass region. When the electrons
and positrons collided with an energy corresponding to the Y(4260) mass,
the Y(4260) could be formed, and this, in fact, led to its initial
discovery.

"In contrast, the BESIII Experiment exploits the unique energy range that
is accessible at the Beijing Electron Positron Collider to produce the
Y(4260) directly and more efficiently by tuning its beam energies to
exactly match the Y(4260)'s mass", Prof. Yangheng Zheng, from the
University of Chinese Academy of Sciences and a co-spokesperson of the
BESIII experiment, said. In the first two weeks of this program, BESIII
had already collected the world's largest sample of Y(4260) decays. Then,
by the end of the first month, evidence pointing to the existence of the
Zc(3900) was already very strong.

The anomalous particles of charmonium, such as the Y(4260) and, now, the
Zc(3900), appear to be members of a new class of recently discovered
subatomic particles, called the XYZ mesons, that are adding new dimensions
to the study of the strong forces that quarks and antiquarks exert on each
other. In the most widely accepted theory of these forces, Quantum
Chromodynamics (QCD), there are in fact more possibilities for charmonium
mesons than simply a charmed quark bound to an anti-charmed quark. Some
theories predict that gluons, the particles that mediate the forces
between quarks, may themselves exist inside mesons in an excited state, a
configuration referred to as "hybrid charmonium." Another proposed
possibility is that more than just a charmed and anti-charmed quark may be
bound together to form "tetraquark" or molecule-like mesons.

In principle, QCD could be used to determine the properties of these more
exotic configurations. The problem is that when QCD is applied to
situations like these, the equations that ensue are impossible to solve,
at least not by normal techniques. Some progress has been made recently
using numerical methods with very high-powered computers to solve the
applicable QCD equations, and there are indications that these methods,
referred to as "lattice QCD," may ultimately be able to account for the
existence of the Y(4260) as a state of hybrid charmonium.

However, the hybrid picture cannot explain the newly discovered Zc(3900),
which decays into an electrically charged pi-meson plus a neutral J/psi
and, thus, must itself carry an electric charge. Since it decays to a
J/psi, the Zc(3900), which has a mass slightly higher than that of a
helium atom, must contain a charmed quark and an anti-charmed quark. But
this configuration is electrically neutral. Adding a gluon to form a
hybrid does not help, because gluons are also electrically neutral. In
order to have a non-zero electric charge, the Zc(3900) must also contain
lighter quarks. Different theoretical models have been proposed that
attempt to explain how this could come about. The positively charged
Zc(3900) particle could be a tightly bound four quark composite of a
charmed and anti-charmed quark plus an additional up quark and anti-down
quark. Or, perhaps, the Zc(3900) is a molecule-like structure comprised
of two mesons, each of which contain a charmed quark (or anti-charmed
quark) bound to a lighter anti-quark (or quark). Another scenario is that
the Zc(3900) is an artifact of the interaction between these two mesons.
In any case, the appearance of such an exotic state in the decay of
another exotic state was unanticipated by most researchers.

At present, the ball is clearly in the experimenters' court and there is
much hope, by theorists and experimenters alike, that with more
experimental data, the veil that continues to shroud these mysterious
particles can be lifted. "We are very excited about this", commented
Yifang Wang, Director of the Institute of High Energy Physics at Beijing.
"With our Beijing collider, we can accumulate a lot more data that will
permit more comprehensive investigations of the nature of this unusual,
electrically charged charmonium state. When all of these results are used
as inputs to theory, we may begin to open the door toward a fuller
understanding of the XYZ particles discovered in recent years."

-The Beijing Spectrometer (BESIII) experiment at the Beijing Electron
Positron Collider is composed of about 350 physicists from 50 institutions
in 11 countries.